Here at work we have a type of oil well component, for example. Each component can be assigned a model designation beginning with C, so we call them "C-types." If I was referring to one, I'd call it "a C-type." C is pronounced "see," which of course begins with a consonant, so it takes the article "a."

To contrast: Today I filed my taxes; although I had some difficulty with the forms, I called "an IRS agent" to get help.

This reminds me of the posts about en and em dashes. We say "an em dash" because the "em" begins with a vowel sound.

The same rule applies to S and X, respectively "ess" and "eks." It's "an SMS" and "an X-ray" (preferably with the hyphen, incidentally).

Do not confuse this with acronyms spoken as words. We still say "a NASA scientist" (NASA is always pronounced as "nassa," here in Houston and elsewhere) and "a SETI investigation" (Search for Extra-Terrestrial Intelligence program, pronounced as "setty").

I was confused on the S because I see that Iranian and Spanish peoples have difficulty pronuncing the words which beging with S following a consonant, such as speak. They'd pronunce "Espeak". So I was wonder if the name of the letter S is "ess" or not.

Hungarian does it so often it's a well-known rule; there are no occurrences of two consonants together at the beginning of a word in the language, so that when they borrow a word, for instance "schola" (school), a vowel sound is added and the word beomes, in this case, "iskola." Since doing so has the effect of putting the two consonants in separate syllables, it may be the case that Hungarian dislikes two consonants together in the same syllable, but I'm not sure if this generalized form of the rule applies.

I've always thought it was curious that Spanish has that E before so many S words (though I love the sound of it). I wonder if it doesn't have something to do with a standard change that happens to a word borrowed from Latin. A Spanish-speaking design drafter where I work thinks I'm on to something here.

If a word starts with a vowel SOUND (not a vowel) then use an. Several consonants in English have names that start with vowel sounds: F (eff), H (aitch), L (ell), M (em), N (en), R (arr), S (ess), and X (eks). As someone has already noticed, the name of the vowel U (yu) starts with a consonant, so it is "a U". Words beginning in U but starting with the Y-sound also take a: a uniform, a university, but an umbrella.

In English an abbreviation might be pronounced like a word (RADAR, NASA) in which case it is called an acronym. It might be pronounced as a group of letters (SMS, DVD) in which case it is called an initialism.

So the article for the same abbreviation may change depending on how the person writing it pronounces it!

Actually, a or an is usually optional before a word beginning in 'H'.a halibut or an halibut are equally correct, although an halibut is mcuh less common, at least in the USA.

and for acronyms, there are many that are both pronounced as a single word and are also spelled out letter by letter. You could use "a" or "an" interchangeably, your choice (at least in writing). Funny, I can't think of any examples right now.